FEBRUARY
1993

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

02-25-93

Plants, Insects Wage Chemical Warfare

Plants are a favorite food of insects. Over 40 percent of all insect species depend on plant material for nourishment. Some chew leaves. Others suck sap. Insects also feed on roots, bore in stems, mine leaves and are called by such names as rootworms, stem borers and leaf miners.

No plants or plant parts are immune to becoming a meal for an insect, although some plants fight back and try to avoid it. Plant defenses against insect feeding include physical barriers such as hairs or thorns.

When it comes to the battle of the bugs, plants also have resorted to chemical warfare. Oak trees, for example, are filled with chemicals called tannins, bitter compounds that keep many insects and other animals from feeding. Insect antifeeding compounds are common in plants. The coffee plant has a high level of caffeine in leaves and berries. Morphine is found in the opium poppy. Quinine comes from the cinchona plant.

Many of the chemicals used by plants for defense have been found by humans to have mind-altering effects. For instance the hemp, or marijuana, plant produces cannabidiol; the coca plant provides cocaine; and the peyote plant supplies mescaline.

Some of these plant compounds are real killers. Strychnine from the strychnos plant is a deadly poison. Other plant poisons have been used to kill insects. Plant-derived insecticides include rotenone from the roots of the derris plant, sabadilla from the seeds of a lily and pyrethrum from the flowers of a chrysanthemum. And, of course, nicotine comes from the leaves of the tobacco plant.

Milkweed plants are not fed upon by most insects and other animals because of a compound known as cardiac glycoside. This compound was so named because humans discovered that it is toxic to the mammal heart. However, when taken at low doses, the chemical can correct some heart ailments and has been used as a heart medicine.

The age-old battle between plants and insects is a no-holds-barred affair — a matter of survival for plants and insects. It is all out chemical warfare. However we humans aren't content to let the plants and insects battle it out. We deliberately expose ourselves to some insect antifeeding compounds like nicotine and caffeine. Who says humans are smarter than insects?

 

Writer: Tom Turpin
Editor: Andrea McCann